1. Field of the Invention
The present invention relates to a high strength steel sheet, and particularly relates to a high strength steel sheet to be a parent material (raw material) of a high strength galvanized steel sheet or a high strength alloyed, galvanized steel sheets.
2. Description of Related Art
A structural member such as member or pillar that operates to absorb energy of crash as a framework member of automobile is required to have high strength in the light of improving safety, or achieving lightweight of the body for improving fuel consumption as a measure of environmental issues. The structural member is further required to have an improved rust prevention property, and a high strength galvanized steel sheets formed by galvanizing a surface of a high strength steel sheet, or a high strength alloyed, galvanized steel sheet formed by being subjected to such galvanization and then alloying, is used as the structural member.
A raw material of such a structural member is required to have formability (elongation) to be formed into a desired shape of a structural member, in addition to the high strength and the rust prevention property. However, since increase in strength of a steel sheet induces reduction in formability, the material is required to balance strength and formability (improvement in balance between strength and elongation).
As a technique for improving formability when a galvanized steel sheet is increased in strength, JP-B-62-40405 describes that a metal structure of a steel sheet can be a mixed structure including a second phase mainly including a ferrite matrix and martensite. However, since strength of the steel sheet disclosed in the literature is only about 600 MPa, further increase in strength is required.
On the other hand, JP-A-9-13147 describes a high tensile alloyed galvanized steel sheet having improved formability and strength of 800 MPa or more. The literature describes that 0.4% or more of Si is added to increase in strength of the steel sheet, in addition, obtain a dual phase structure of ferrite and martensite as a metal structure of a steel sheet. However, a relationship between Si and balance of strength with elongation is not noted, and therefore the balance between strength and elongation has been sometimes degraded.